CMP
Chemical-mechanical polishing and chemical-mechanical planarization, both of which are referred to in the art as “CMP”, are abrasive techniques that typically include the use of a combination of chemical and mechanical agents to planarize, or otherwise remove material from or planarize a surface of a semiconductor material substrate during the fabrication of devices thereon. A chemical component, typically a slurry that includes one or more oxidizers, abrasives, complexing agents, and inhibitors, oxidizes the surface of one or more material layers that are being polished or planarized (i.e., at least partially removed). A polishing pad formed from a material such as polyurethane or acrylic is used with the slurry and, in combination with abrasives present in the slurry, effects mechanical removal of the layer or layers from the surface of the semiconductor device structure. It should be noted that abrasive-only polishing and planarization, e.g., without the use of active chemical agents to effect material removal, are becoming more prevalent due to environmental concerns. Thus, the term “CMP” as used herein encompasses such abrasive-only (i.e., strictly mechanical) methods and apparatus.
Copper Conductive Structures
The use of copper as a conductive material in semiconductor devices is also ever increasing. When copper is used in semiconductor devices, however, a barrier layer is typically required between the copper and adjacent structures or layers. The barrier layer prevents diffusion of the copper into the adjacent layers or structures, as well as the formation of copper silicides, both of which may cause electrical shorts in semiconductor devices that include copper. Tantalum is an example of a material that is useful as a copper barrier. When tantalum is used, the semiconductor device, including any features thereof into which copper is to be disposed (e.g., trenches), is lined with a layer of tantalum. The tantalum layer is then typically covered with a thin copper layer, often formed by physical vapor deposition (“PVD”) processes. The thin copper layer then acts as a so-called “seed layer” for the formation of a copper structure, such as a conductive line, such as by electroplating processes.
Once the tantalum and copper layers have been formed, it is necessary to isolate separate tantalum-copper conductive structures from one another. CMP processes are typically used to remove the tantalum and copper between the structures from over the active surface of the semiconductor device being fabricated. Slurries that are used in copper CMP processes typically have a pH of about 7.0. Many of these slurries include hydrogen peroxide (H2O2) as an oxidizing agent. Since hydrogen peroxide readily generates hydroxy free radicals (OH), hydrogen peroxide is a very strong oxidizing agent. Tantalum, however, is substantially chemically inert. Thus, the oxidizers of CMP slurries that remove copper do not effectively oxidize tantalum and, thus, do not adequately effect the removal of tantalum. Likewise, slurries that are useful for removing tantalum by CMP processes are likewise not effective for removing copper. As a result, when conventional CMP processes are used to isolate the tantalum-copper conductive structures of a semiconductor device, two separate slurries must be used.
It has been proposed that tungsten be used in place of tantalum in semiconductor devices as a barrier material for copper conductive structures. Nonetheless, when known copper CMP slurries are used to substantially simultaneously CMP tungsten and copper, the tungsten barrier layer may dissolve, or be removed, at a faster rate than the copper. This is at least partially because, as the following chemical equations illustrate, tungsten (W) is more readily oxidized than copper (Cu):W+2H2O→4H++4e−+WO2 Eo=0.12;Cu→Cu2++2e−Eo=−0.34.
Thus, in conventional slurries, although both copper and tungsten are simultaneously exposed to the same oxidants, the tungsten will typically be oxidized first. As a result, gaps may form in locations where the barrier material should be located between copper conductive structures and adjacent portions of the semiconductor device structure upon which the conductive structures are being fabricated.
This phenomenon is illustrated in the electron micrograph of FIG. 1, which illustrates a semiconductor device structure 10 that includes the portions of a copper layer 20 and an underlying tungsten barrier layer 18 disposed within a recess 14 formed in an active surface 16 of a substrate 12 of semiconductor device structure 10 following CMP thereof using an alumina fixed-abrasive polishing pad and a copper CMP slurry having a pH of about 7. Once an interface 19 between barrier layer 18 and copper layer 20 was exposed during the CMP process, tungsten of barrier layer 18 was oxidized and dissolved at a faster rate than the adjacent copper of copper layer 20, leaving a gap 21 between copper layer 20 and adjacent regions of substrate 12, as well as undesirably permitting copper of copper layer 20 to contact and, possibly, diffuse into, unprotected adjacent regions of substrate 12.
Therefore, it would is desirable to provide a slurry that is useful in CMP processes and that effectively polishes or planarizes both copper and tungsten without causing oxidation or dissolution of the tungsten.